Deterministic Leak-Before-Break Treatment of Uncertainties: Part 1 – Theoretical Basis

Abstract The Canadian CANDU® Industry has developed and is implementing a composite analytical approach (CAA) to demonstrate, with high confidence, appropriate safety margins for deterministic nuclear safety analysis of large-break loss-of-coolant accident events in existing CANDU reactors. A key element of the CAA is the deterministic leak-before-break (CAA/DLBB) assessment for postulated through-wall cracks in all butt welds in the large-diameter primary heat transport system piping and reactor headers. This paper describes a systematic approach for quantifying the uncertainty in the CAA/DLBB assessment. The proposed approach integrates the uncertainties of the key leak-rate parameters into a single metric (a quantitative indicator of the uncertainty in the CAA/DLBB assessment, a.k.a., performance indicator). This approach ensures that the uncertainty in individual key parameters is not taken out of the context of the uncertainty of the other key parameters. A brief description of the CAA/DLBB evaluation process is presented and the leak-rate factor is adopted as the figure of merit. Next, the key leak-rate effect factors are identified and the uncertainty in each is established. This is followed by a description of two statistical approaches that can be used to propagate the uncertainties for the bounding postulated break location and to determine the level of uncertainty associated with the CAA/DLBB assessment. In Part 2 of the paper, the application of the approach is illustrated.